This application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 2006-08676, filed on Jan. 27, 2006 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to communication systems, and more particularly, to adaptive demodulation of a PWM signal by adjusting capacitance and discharge current in a delay filter.
2. Background of the Invention
In general, radio frequency identification (RFID) transponders or RFID tags began to be studied for identifying livestock early in the United States of America. Demand for RFID systems have increased for other applications such as using an RFID tag for a smart card (chip card or integrated circuit card) for providing convenience and security.
For example, unauthorized access to personal information in smart cards used as credit or financial cards is desired to be prevented. As technology for forging and hacking such private cards improves, the conventional magnetic card is desired to be changed to a chip card (or IC card) especially for use as financial cards. Application of RFID tags is extending even to pricing models and logistics systems in contact with mobile communication systems, as well as to smart card systems.
For contact-less smart cards, the RFID tag embeds identification codes that are internally recognizable. When a reader deciphering such codes sends an RF signal to the RFID tag, the identification codes stored in the tag are transferred to the reader by way of a modulator comprised in the tag. The RFID tags for the contact-less smart cards use power from current induced at coils by RF signals from the reader.
One of general communication modes is pulse-width modulation (PWM) that transforms binary data into patterns of pulse width. For instance, in an RFID tag operable in 900 MHz, when a pulse width of a received signal is ⅛ of a pulse period T0, the received signal is demodulated into a logic low level [0]. Otherwise, when the pulse width is ⅜ of the pulse period T0, the received signal is demodulated into a logic high level [1]. For such demodulation, the RFID tag includes a demodulator operating with delay filtering.
FIG. 1 is a circuit diagram of a conventional PWM demodulator with delay filtering in a smart card. Referring to FIG. 1, the PWM demodulator receives and inverts a PWM (pulse width modulation) signal to generate an inverted PWM signal RxData_filter. The inverted PWM signal RxData_filter charges or discharges a capacitor C having a fixed capacitance. The inverted PWM signal RxData_filter that is at the logic high level [1] charges the capacitor C, and that is at the logic low level [0] discharges the capacitor C.
The PWM demodulator of FIG. 1 includes a comparator 20 that generates a high logic level [1] when the voltage at the capacitor C is charged to higher than a reference voltage Vref. Thus, when the received PWM signal RxData_org has a period with the logic low level for (⅜)T0, the comparator 20 outputs the logic high level [1]. When the received PWM signal RxData_org has a period with the logic low level for (⅛)T0, the comparator 20 outputs the logic low level [0]. Thus, the comparator 20 generates digital output RxData_fin. FIG. 2A shows timing diagrams of such example signals RxData_org, RxData_filter, and RxData_fin during operation of the PWM demodulator of FIG. 1.
FIG. 2B shows timing diagrams of signals during operation of a PWM demodulator that samples the received PWM signal RxData_org using a clock signal generated by an internal oscillator, in contrast to the delay-filtering type PWM demodulator of FIG. 1. In that case, the inverted PWM signal is output as sampled pulses RxData_sample. Such sampled pulses RxData_sample may be counted for generating the binary output RxData_fin.
Unfortunately, the internal oscillator used in such a PWM demodulator increases power consumption and causes fluctuation of internal power supply. Such a fluctuation results in instability of the sampling frequency, and thus in inaccurate demodulation of the PWM signal. Therefore, the delay-filtering type PWM demodulator is desired in a contact-less RFID tag.
However, the delay-filtering PWM demodulator of FIG. 1 uses the capacitor C having a fixed capacitance. The capacitance of the capacitor C may not be predictable because of process variations during fabrication of the delay-filtering PWM demodulator of FIG. 1 as an integrated circuit. Furthermore, the delay-filtering PWM demodulator of FIG. 1 using the fixed capacitor C may not operate properly in various RF environments with variation of the reader and fluctuation of power transmitted through RF.